In general, an OLED display is a display device that electrically excites fluorescent organic material for emitting light and visualizes an image by voltage programming or current programming N×M organic light emitting pixels.
An organic light emitting pixel (or diode) includes anode (indium tin oxide or ITO), organic thin film, and cathode (metal) layers.
The organic thin film layer has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) so as to balance electrons and holes to thereby enhance light emitting efficiency. Further, the organic thin film separately includes an electron injection layer (EIL) and a hole injection layer (HIL).
Methods of driving the organic light emitting pixels having the foregoing configuration include a passive matrix method and an active matrix method employing a thin film transistor (TFT) or a MOSFET.
In the passive matrix method, an anode and a cathode are formed crossing each other, and a line is selected to drive the organic light emitting pixels. In the active matrix method, an indium tin oxide (ITO) pixel electrode is coupled to the TFT, and the light emitting pixel is driven in accordance with a voltage maintained by capacitance of a capacitor.
Herein, the active matrix method can be classified as a voltage programming method or a current programming method depending on the type of signals transmitted to the capacitor so as to distinctively control the voltage applied to the capacitor.
A pixel circuit according to a conventional voltage programming method has difficulties in expressing high-level grayscales due to deviations of threshold voltages VTH of TFTs and/or mobilities of carriers of the TFTs, the deviations being generated as a result of a non-uniform manufacturing process of the TFTs.
On the other hand, although currents and/or voltages supplied from driving transistors in a plurality of pixel circuits may not be uniform, a pixel circuit employing a current programming method can provide panel uniformity as long as a current supplied from a current source to the pixel circuits is uniform.
In realization of a display device by using the pixel circuit that employs the current programming method, a digital/analog (D/A) converter is required to convert grayscale data into a grayscale current so as to apply the grayscale current to the pixel circuit. In addition, the D/A converter performs a gamma correction on the grayscale data in consideration of characteristics of a display panel of the display device.
However, a conventional D/A converter outputs linear grayscale currents corresponding to grayscale data so that the conventional D/A converter cannot satisfy non-linear gamma characteristics of a display panel.
Accordingly, a desired image is not displayed on the display panel and thus image quality is degraded.
The above information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgment or any form of suggestion that the above information forms the prior art that is already known in this country to a person skilled in the art.